Boeing’s newest and largest 737 Max gunned down the runway and soared over the shore of Lake Washington, south of Seattle, on its maiden flight Thursday.

The latest model in Boeing’s best-selling jet family achieved the critical milestone at 10:52 a.m. local time, days after the Chicago-based planemaker celebrated the 50th anniversary of the first 737 flight. The single-aisle Max 9 faces an uncertain future, however, in a market dominated by Airbus’s longer A321neo.

The U.S. planemaker is trying to pull off a balancing act with the 737, a workhorse favored by budget carriers that is Boeing’s largest source of profit. The manufacturer is mulling the introduction of as many as five Max models targeting different niches, while ratcheting up the tempo in its Renton, Washington, factory over each of the next three years.

Any stumbles in developing the new jets, a process fraught with delays, could damage Boeing’s bottom line if snarls slow manufacturing at a plant preparing to increase output to 47 planes a month in May, five more than the present rate. The Max 8, the first of the upgraded 737 models, has completed flight-testing and is slated to begin deliveries this quarter, months ahead of schedule.

The Max 9 also took to the skies ahead of schedule, with Captain Christine Walsh in command. It is designed to seat 204 travelers in a single-class cabin, about 16 more people than the Max 8, while flying the same distance: as many as 3,515 nautical miles.

The debut is the latest in a year crammed with new planes produced by manufacturers from Brazil to Ukraine. Boeing’s largest 787 Dreamliner, the -10, took its first flight March 31 — the same day that Airbus’s A319neo and Antonov’s An-132D turboprop aircraft made their maiden flights.

Sputtering airplane sales raise concerns that the aerospace industry is headed into a downturn after more than a decade of growth, making it tougher for manufacturers to recapture the billions of dollars spent developing aircraft.

“This is why we have a duopoly,” said aerospace analyst Richard Aboulafia, referring to Boeing and Airbus. “Newcomers have a harder time with an ultimately cyclical market, combined with heavy spending on new-product development.”

Boeing shouldn’t have any problem recovering its costs for the 737 Max family, the latest versions of the venerable single-aisle jet that revolutionized low-cost travel and aircraft manufacturing. The company has already landed 3,703 orders, the most in its history for a new program. Europe’s Airbus has garnered 5,056 orders for its A320neo lineup.

The bulk of Boeing’s orders are thought to be for the -8, which seats more travelers than the A320neo, although the U.S. planemaker doesn’t break out sales by Max model. That gives Airbus a commanding lead at the top end of the market with the 1,385 orders notched by the A321neo, the longest narrow-body in production.

With the Max 9 struggling to gain sales, Boeing has begun marketing the Max 10X, a longer model. It would seat about 230 travelers and cruise on transcontinental routes flown by Boeing’s 757, which has been out of production for more than a decade.

But potential customers, like Air Lease Corp., fret that the new model will get to market too late, with a planned 2020 debut. That will be a year after Airbus will introduce an A321neo model configured to seat as many as 240 people. Boeing would probably have to discount heavily to cut into Airbus’s lead, Aboulafia said.

“They can’t seem to get the pricing power they had with the NG series,” the analyst said, referring to the previous generation of the 737s that commanded premiums to Airbus jets. “Maybe that will change. But if it does, it will happen on the Max 8.”

But Boeing, for now, may be content to have product offerings that it can hone for 737 operators — without ceding the top end of the narrow-body market to Airbus, said Ken Herbert, an analyst at Canaccord Genuity.

“The investments haven’t been excessive relative to the broader program,” he said. The Max 9 and 10X will help “keep Airbus pricing in check,” he said. “Part of the benefit is just having something in place and letting the technology mature.”